Power generating systems using fuel cells for the practical uses have been studied for several ten years. The operating temperature of a fuel cell is determined primarily by the temperature at which ionic conduction occurs in the electrolyte layer. Among others, a fuel cell using a solid oxide electrolyte has a high operating temperature and may achieve high power generating efficiency.
A fuel cell battery is constructed by connecting many power generating cells in series or in parallel. Since variations in power generating characteristics exist among the individual power generating cells, when generating power by connecting many power generating cells the operating temperature for power generation is set for each power generating cell in order to stably operate each power generating cell at its optimum power output. Therefore, in a fuel cell battery, it is desired to control the temperature of each power generating cell.
Further, the operating temperature of the type of fuel cell that uses a solid oxide electrolyte is as high as, for example, 500° C. to 1000° C. When operating the fuel cell at rest, it is desired to reduce the time necessary for the fuel cell to reach its operating temperature after it has been started up.
There is therefore a need to quickly raise the temperature of each power generating cell of the fuel cell battery or to individually control the temperature of each power generating cell.
In the power generating operation of the fuel cell that uses an exothermic reaction, it is also desirable to effectively utilize the thermal energy produced during the power generation.
In a large-scale fuel cell system, the thermal energy produced during the power generation may be effectively utilized, for example, by using the energy to heat water and thus use it as hot water or to operate a generator using a mechanical thermal engine such as a turbine as a means for converting the energy to electric energy. However, since such mechanical means have many mechanical moving parts, the structure is complex and maintenance is not easy, and in the case of a small-scale fuel cell system, using mechanical means such as a turbine may not be suitable.
Japanese Laid-open Patent Publication No. 2005-228523
Japanese Laid-open Patent Publication No. 2007-200710